192 PROCEEDINGS OP THE AMERICAN ACADEMY 



A,. :^ polar distance of Venus = PV. 



a's = polar distance of the reflected image of the true sun = PS'. 



A'y = polar distance of the reflected image of Venus = PV. 



tg = hour angle of true sun = APS. 



ty = hour angle, of Venus = APV. 



t's = hour angle of the reflected image of the true sun = APS'. 



t'y = hour angle of the reflected image of Venus ^ APV. 



^g = zenith distance of true sun = ZS. 



^'s =: zenith distance of the reflected image of the true sun = ZS'. 



^"s = zenith distance of the reflected image of the apparent 



sun = ZS'^. 

 t,'^, = zenith distance of the reflected image of Venus = ZV. 

 As = azimuth of the true sun = AZS. 



A's = azimuth of the reflected image of the true sun = AZS'. 

 A"s = azimuth of the reflected image of the apparent sun = AZS'^. 

 A'v = azimuth of the reflected image of Venus = AZV. 

 d = the angle ZS'^S'. 

 r =. the refraction. 



7i' = the parallax in altitude. Hence r — tt' = SS^ = S'S'^. 

 § = the angle ZS'V. 

 7 = the angle ZV'S'. 

 X = the angle PS'V. 

 Q =. the geocentric distance from centre of sun to centre of 



Venus = S V = SV. 

 CO = the geocentric position angle of Venus relatively to the sun's 



centre = PSV. 

 a z= the angle PVS. 

 ^ =. the angle PSS'. 

 yj = the angle PS'S. 



We have next to show how pictures obtained vpith the horizontal 

 photoheliograph may be measured, and the results expressed in terms 

 of some one of the systems of spherical co-ordinates usually employed 

 in astronomy. 



Upon an engine arranged to give polar co-ordinates, the plate to be 

 measured is carefully adjusted, so that the centre of the image of the 

 sun coincides as accurately as possible with the origin of co-ordinates. 

 The fixed line, from which the angles of the co-ordinates are to be 

 measured, is taken perpendicular to the image of the plumb line ; the 

 zero being toward that edge of the plate which was on the right hand, 

 as viewed from the object glass, when the picture was taken, and (the 



